The present invention relates to an image forming apparatus such as a facsimile machine, a digital copying apparatus or the like.
A known image forming apparatus includes a corona charger for uniformly charging a photosensitive surface of a photosensitive member, a latent image forming device for forming on the charged photosensitive surface, a latent image corresponding to recording data a developing device for developing the latent image into a visible image and a transfer device for transferring the visible image onto a recording paper sheet such that image formation is sequentially performed by displacing the photosensitive surface. At this time, quality of the image reproduced on the recording paper sheet largely depends on uniformity of charging of the photosensitive surface and stability of density of developer in the developing device. Thus, it is considered effective for improving quality of the reproduced image to monitor density of developer.
FIG. 1 shows one example of the known image forming apparatus. The known image forming apparatus includes a semiconductor laser 11, a rotary polygon mirror 12, an f.theta. lens 13, a second mirror 14, a first mirror 15, a cleaner 16, a corona charger 17, a photosensitive member 18, a color developing device 19, a photosensor 110, a light emitting element 111, a toner density detecting circuit 113, a pattern generator 114, a reader 116 for reading an original document, a laser modulation circuit 117, a toner density detecting means 91, a controller 92, a control circuit 93 for controlling laser current and a control circuit 94 for controlling developing bias. The toner density detecting means 91 includes the photosensor 110, the light emitting element 111 and the toner density detecting circuit 113. In FIG. 1, reference numeral 115 denotes a normal relative to an outer peripheral surface of the photosensitive member 18.
Operation of the known image forming apparatus of the above described arrangement is described with reference to FIGS. 2 to 4. Initially, a control signal f outputted from the controller 92 is inputted to the pattern generator 114. Thus, a pattern signal i is produced in the pattern generator 114 so as to be outputted to the control circuit 93. Meanwhile, a laser current setting signal k for setting laser current is preliminarily applied to the control circuit 93 from the controller 92. In response to the laser current setting signal k from the controller 92, the control circuit 93 determines a value of current for driving the semiconductor laser 11. The control circuit 93 outputs, in turn, a laser current controlling signal 1 and a pattern signal j to the laser modulation circuit 117. In response to the laser current controlling signal 1 and the pattern signal j, the laser modulation circuit 117 produces a modulation signal e for modulating the semiconductor laser 11 and outputs the modulation signal e to the semiconductor laser 11 such that the semiconductor laser 11 is modulated by the modulation signal e.
Meanwhile, the photosensitive member 18 is preliminarily charged by the corona charger 17 uniformly. In response to the modulation signal e from the laser modulation circuit 117, a plurality of patterned latent images are formed on the photosensitive member 18 by the semiconductor laser 11. Then, a control signal h for controlling developing bias is outputted to the control circuit 94 from the controller 92. In response to the control signal h from the controller 9&, the control circuit 94 outputs developing bias voltage signals m, n, o and p to the color developing device 19 such that the patterned latent images formed on the photosensitive member 18 are sequentially developed, through adhesion of toner thereto, into visible patterned images 101 (FIG. 2) by the color developing device 19 as will be described in detail later. Toner density of the visible patterned images 101 is detected by the toner density detecting means 91.
In response to a light emitting signal b from the toner density; detecting circuit 113, the light emitting element 111 irradiates light over the visible patterned images 101 at an incident angle .theta..sub.0 relative to the normal 115 as shown in FIG. 3. The irradiated light is reflected by toner particles 22 (FIG. 3) of the visible patterned images 101 such that this reflected light is detected at an angle .theta..sub.1 of reflection relative to the normal 115 by the photosensor 110. At this time, in order to detect direct reflected light from the photosensitive member 18, the incident angle .theta..sub.0 is so set as to be identical with the angle .theta..sub.1 of reflection, i.e. .theta..sub.0 =.theta..sub.1. A quantity a of direct reflected light detected by the photosensor 110 is transmitted to the toner density detecting circuit 113 and thus, the toner density detecting circuit 113 outputs a detection signal c to the controller 92. In the controller 92, the detection signal c is compared with a preset reference value and the laser current setting signal k is calculated so as to obtain optimum density characteristics. The controller 92 outputs the laser current setting signal k to the control circuit 93 so as to optimize a value of current for driving the semiconductor laser 11.
Since the control signal h for optimizing the developing bias voltage signals m, n, o and p applied to the color developing device 19 is outputted to the control circuit 94 from the controller 92 as described above, it is possible to control voltages applied to the color developing device 19. Thus, by employing at least one of the laser current controlling means (control circuit 93) and the developing bias controlling means (control circuit 94), density correction for each color can be performed.
After completion of density correction, the visible patterned images 101 are removed from the photosensitive member 18 by the cleaner 16. In accordance with an image signal g applied to the controller 92 from the reader 116, the modulation signal e is produced by the laser modulation circuit 117 and an image of the original document subjected to density correction is outputted through developing, transfer and fixing steps in a known electrophotographic process.
FIG. 2 shows the visible patterned images 101 formed on the photosensitive member 18[In response to the modulation signal e from the laser modulation circuit 117, the visible patterned images 101 are formed through rotation of the photosensitive member 18 in the direction of the arrow A such that amount of toner adhering to each of the visible patterned images 101 increases gradually as the photosensitive member 18 is further rotated in the direction of the arrow A.
FIG. 3 shows the known toner density detecting means 91. In FIG. 3, an optical axis 26 of the light emitting element 111 forms the angle .theta..sub.0 with the normal 115 on the outer peripheral surface of the photosensitive member 18, while an optical axis 27 of the photosensor 110 forms the angle .theta..sub.1 with the normal 115 on the outer peripheral surface of the photosensitive member 18. The light emitting element 111 irradiates light over the toner particles 22 of the visible patterned images 101, which adhere to the photosensitive member 18. Irradiated light from the light emitting element 111 is reflected by the photosensitive member 18. At this time, quantity of reflected light changes according to amount of the toner particles 22 adhering to the photosensitive member 18. As the amount of the toner particles 22 adhering to the photosensitive member 18 is increased, quantity of light reflected by the photosensitive member 18 decreases. This reflected light is received by the photosensor 110 set at the angle .theta..sub.1 relative to the normal 115. The angles .theta..sub.0 and .theta..sub.1 of the light emitting element 111 and the photosensor 110 are, respectively, so set as to be equal to latch other such that detection of direct reflected light on the photosensitive member 18 is facilitated.
FIG. 4 shows output characteristics of the photosensor 110 in the toner density detecting means 91. It will be seen from FIG. 4 that output voltage of the photosensor 110 assumes 3.9 V when the toner particles 22 do not adhere to the photosensitive member 18. Meanwhile, output voltage of the photosensor 110 decreases as amount of the toner particles 22 adhering to the photosensitive member 18 is increased. Since output voltage of the photosensor 110 assumes 3.5 V when a maximum amount of the toner particles 22 adhere to the photosensitive member 18, dynamic range of output voltage of the photosensor 110 is 0.4 V.
However, in the above described known arrangement, since toner density is read from reflected light on the photosensitive member 18, dynamic range of output voltage of the photosensor 110 is small and signal-to-noise ratio is poor, so that it is impossible to accurately control density gradation of an outputted image.